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Revert "cpufreq: Drop rwsem lock around CPUFREQ_GOV_POLICY_EXIT" 

Earlier, when the struct freq-attr was used to represent governor
attributes, the standard cpufreq show/store sysfs attribute callbacks
were applied to the governor tunable attributes and they always acquire
the policy->rwsem lock before carrying out the operation.  That could
have resulted in an ABBA deadlock if governor tunable attributes are
removed under policy->rwsem while one of them is being accessed
concurrently (if sysfs attributes removal wins the race, it will wait
for the access to complete with policy->rwsem held while the attribute
callback will block on policy->rwsem indefinitely).

We attempted to address this issue by dropping policy->rwsem around
governor tunable attributes removal (that is, around invocations of the
->governor callback with the event arg equal to CPUFREQ_GOV_POLICY_EXIT)
in cpufreq_set_policy(), but that opened up race conditions that had not
been possible with policy->rwsem held all the time.

The previous commit, "cpufreq: governor: New sysfs show/store callbacks
for governor tunables", fixed the original ABBA deadlock by adding new
governor specific show/store callbacks.

We don't have to drop rwsem around invocations of governor event
CPUFREQ_GOV_POLICY_EXIT anymore, and original fix can be reverted now.

Fixes: 955ef48335 (cpufreq: Drop rwsem lock around CPUFREQ_GOV_POLICY_EXIT)
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reported-by: Juri Lelli <juri.lelli@arm.com>
Tested-by: Juri Lelli <juri.lelli@arm.com>
Tested-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
Viresh Kumar 2016-02-09 09:01:35 +05:30 committed by Rafael J. Wysocki
parent fd8ddc482a
commit 68e80dae09
2 changed files with 0 additions and 9 deletions
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5
drivers/cpufreq/cpufreq.c
View File

@ -2205,10 +2205,7 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy,
return ret;
}
up_write(&policy->rwsem);
ret = __cpufreq_governor(policy, CPUFREQ_GOV_POLICY_EXIT);
down_write(&policy->rwsem);
if (ret) {
pr_err("%s: Failed to Exit Governor: %s (%d)\n",
__func__, old_gov->name, ret);
@ -2224,9 +2221,7 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy,
if (!ret)
goto out;
up_write(&policy->rwsem);
__cpufreq_governor(policy, CPUFREQ_GOV_POLICY_EXIT);
down_write(&policy->rwsem);
}
/* new governor failed, so re-start old one */

4
include/linux/cpufreq.h
View File

@ -100,10 +100,6 @@ struct cpufreq_policy {
* - Any routine that will write to the policy structure and/or may take away
* the policy altogether (eg. CPU hotplug), will hold this lock in write
* mode before doing so.
*
* Additional rules:
* - Lock should not be held across
* __cpufreq_governor(data, CPUFREQ_GOV_POLICY_EXIT);
*/
struct rw_semaphore rwsem;